A significant amount of that weight is the battery, so making the battery 2.5x heavier would make the phone substantially heavier, even with the same overall charge. You'd be hard pressed to sell someone on 'This new phone we made is great, you get the same battery life as before but it weighs 40% more!'

I suppose the question is, in which cases would people be willing to accept a lower charge capacity if their phone could charge to full in, say, minutes rather than hours? I feel as though those USB battery packs that are everywhere these days would be a great candidate; plug it into the wall for a few minutes and you're fully charged, and you can charge anything else from there.